The increase in the use of rechargeable batteries in recent years has also made it important to make it convenient for consumers to use the batteries by speeding up their recharging. Generally, the battery charging process takes an inconveniently long time and when high currents are employed to quicken the charging process an undesirable battery heating and loss of battery cycle life frequently occurs.
Techniques have been proposed to rapidly charge a battery, see for example U.S. Pat. Nos. 5,307,000 and 5,204,611. The U.S. Pat. No. 5,307,000 patent describes a charging technique wherein a charge pulse is followed by a discharge pulse of much shorter duration followed by a first rest period and the another discharge pulse followed by a second rest period. The U.S. Pat. No. 5,307,000 patent is incorporated herein by reference thereto.
Fast charging processes should, however, have reliable techniques for termination of the process. One proposed method relies upon sensing battery temperature and terminating battery charging when the temperature becomes too high. As a practical matter, however, the temperature warning is sensed too late thus leading to an overheating of the battery. When the charging process uses charging pulses followed by discharge pulses with rest periods in between them particular care should be taken to avoid overheating of the battery.
Another technique for terminating a battery charging relies upon sensing the battery voltage and its negative change. This technique, however, does not provide a reliable indicator of the completion of a charging cycle because the battery voltage is not stable and varies as a function of the battery temperature and charge cycle history. Also, the sensing of a negative battery slope in the battery voltage as a function of time tends to be masked by the same slope effect from battery temperature changes. Reliance upon sensing battery voltage changes while the battery is under load or being charged is a coarse approach and should not be relied upon because of the effects from temperatures or high charging currents.
Another known technique for termination of the charging of a battery is based upon the simple passage of a particular length of time. Such an approach, if it is to work reasonably accurately, depends upon an assumption of a particular state of charge at the start of the charge cycle. Since this is not commonly known, there is a tendency to either over or under charge the battery.
In the charging technique described in U.S. Pat. No. 5,204,611 the battery or cell voltage is sensed during an interval when the charging current is interrupted and is then compared with a preset reference voltage. The comparison signal is then used to control the charging current. The problem associated with this technique is in choosing the right value for the reference voltage. This voltage should be a function of the battery's past charge cycle history, but is fixed at a particular value for practical reasons.
The Russian patent 1129675, which issued Dec. 15, 1984, suggests polarization and depolarization pulses with a rest period charging algorithm. The described technique, however, has disadvantages because the voltage measured during charging to determine when to terminate the charging process varies up to as much as 30%.
Hence, there exists a need for an effective method for terminating the rapid charging of a battery without excessive heating and electrolyte decomposition.